This invention relates generally to a system for shot blasting and separating light particles from heavier particles in a gas stream and in particular to separating paint in the form of flakes, chips and dust from shot blast media.
In shot blasting of painted surfaces it is now essential to avoid the release of contamination to the environment via the used blasting media. The paint frequently contains lead or other toxic or undesirable elements. At the same time, it is essential to protect the shot blast operators and other workers from inhalation of the dust produced during the shot blast operation.
An apparatus for removing matter, such as asbestos, from a gas stream using a displacement chamber for separating coarse and fine material, and a filter system having two regenerative filters for trapping the fines, is disclosed in U.S. Pat. No. 5,064,454. At least two filter units are installed in parallel to receive and filter the gas stream. The filter units have outlets at the bottom for solid matter and first and second valves respectively on either side of each filter unit for cleaning by compressed air when desired. The displacement chamber is upstream of the filter units, with a solids removal opening at the bottom. The bottom outlets of the filters and the displacement chamber are sealed from the surrounding environment, and the filtered solid matter is discharged into bags via at least one bagging unit sealed to the outlets and the discharge opening. Third valves are provided at the solid matter outlet of each filter unit, and these outlets are connected back to the displacement chamber via a return conduit. Fourth valves from the filter units are connected to a higher pressure gas source, such as the atmosphere in a vacuum system, so that the solid matter may be routed back to the displacement chamber by closing the first and second valves and opening the third and fourth valves. A ram compresses the coarse material collected at the bottom of the displacement chamber whilst the fines collected in the filter system are preferably recycled into the bottom of the displacement chamber during cleaning of the filters. If the gas stream contains toxic material, such as toxic paint chips, the toxic material will be recycled from the filters to the displacement chamber until a hazardous concentration of toxic material in the system is reached. The compressed coarse material is bagged for disposal.
The above described system works best for applications where most of the collected material comprises a large volume of toxic material which is to be disposed of, for example asbestos removal. However, this clearly creates a disposal problem, and/or a cost problem, if the total volume of toxic material is small compared to the non-toxic volume of collected material. All of the collected material must be disposed of to an approved waste site, although only a part of the material is toxic. Since both toxic and non-toxic material is mixed in the storage bags, all of the collected material requires special and costly disposal facilities. Furthermore, there is a relatively large quantity of material to be disposed, which in itself increases the cost of disposal.
To allow more than one shot blast hood to be connected to the displacement chamber without loss of vacuum to any of the hoods, a vacuum compensation valve, as disclosed in WO 94/22355 and U.S. Pat. No. 5,566,421, is utilized. Multiple hoses are connected to the displacement chamber for collection of materials from various locations (whether immediately adjacent each other, where the hoses may be ganged together if desirable, or whether at various locations remote from each other). Pressure sensors associated with each hose sense the pressure in the hose and dampers are employed, in response to the relative pressures between the hoses, to vary the effective area of the connection between the hose and the displacement chamber, so as to equalize and optimize the relative pressures. Preferably, conical plungers or stoppers which are movable towards and away from corresponding seats by linear actuators, are used as dampers. The dampers are preferably controlled by a programmable logic controller (PLC) which receives signals from the pressure sensors and is programmed to move the dampers to their optimum positions for the sensed pressures.
When the apparatus described above is used for cleaning an air stream carrying shot blast media, removed paint, rust etc. from a treated surface, the compressed coarse material will contain used shot blast media and coarser paint chips or flakes together with larger rust particles and other material that was shot blasted from the treated surface. An unnecessary large volume of material will have to be disposed of unless the still useable shot blast medium can be separated from the coarse material, such as possibly toxic paint chips, dust and rust flakes, and be reused. It is, therefor, an object of the invention to provide a suitable system for achieving the desired separation, within the context of an overall system which permits the shot blast media to be reused.
A further object of the invention is to provide a suitable system for removing toxic particles from the filter system, before the recycling of material builds up toxicity in the system to a hazardous level.
In the invention, a shot blast hood is connected to a shot blast media feeding hose and the hood is surrounded by a hood, which is connected to a vacuum source to collect all of the shot blast media, paint flakes, dust, etc. from the area where the shot blasting is taking place. The hose which collects the shot blast media, chips, dust, and other debris, is connected to a xe2x80x9cdisplacement chamberxe2x80x9d. Several such hoses may be connected to the chamber, so that shot blasting may take place simultaneously at several different locations. Within the displacement chamber, a baffle is positioned to direct the incoming airstream downwardly, thus directing most of the debris downwardly. However, air is drawn off the top of the displacement chamber via at least one outlet, through high performance filters, by a blower unit which provides negative pressure to the whole system. This air carries most of the dust and xe2x80x9cfinesxe2x80x9d which are particularly hazardous to health.
From the bottom of the displacement chamber, the shot blast material and heavy paint chips, etc. are discharged to a vibratory screen separator via a rotary valve. The vibratory screen separates the paint chips from the shot blast media, so that the shot blast media may be reused. The shot blast media may be reused several hundred times, making the successful separation desirable also for reducing cost. The paint chips are bagged for disposal.
From the displacement chamber, the dust-laden air is drawn to one of two filter units, a first filter unit and a second filter unit. Each filter unit includes several HEPA filters mounted vertically within a housing. Air is drawn through the filters via a duct leading from the clean side of the filters to the main blower unit.
When the filters in the first filter unit start to become clogged, as can be readily detected by gauges which measure the pressure drop across the filters, the first filter unit is taken off-line, and the airstream is directed through the second filter unit. Once the first filter unit is off-line, a valve is opened from the bottom of the first filter unit where much of the dust will have settled, to draw the dust to a paint and dust filter which is connected between the first filter unit and the displacement chamber. The paint and dust filter uses the negative pressure in the displacement chamber to draw the dust from the first filter unit through the paint dust filter. A small air inlet on the filter unit is opened to allow air into the first filter unit so that the dust can be drawn to the paint and dust filter. Once the first filter unit has been cleaned in this manner, the valve to the paint and dust filter is closed, and the dust can be removed from the paint and dust filter via a rotary valve at the bottom.
As the second filter unit becomes clogged, this process is repeated, but this time with the second filter unit being taken off-line and connected to the paint and dust filter while the first filter unit is back on-line.
Further features of the invention will be described or will become apparent in the course of the following detailed description.